Tidal irrigation-based rice cultivation enhances coastal blue carbon and decreases GHG emissions in brackish mudflats

Coastal agriculture has been adopted to increase agricultural productivity, whereas its effects on blue carbon ecosystem function and greenhouse gas (GHG) exchange dynamics are unclear. This research examined the impact of tidal saltwater irrigation on agronomic traits, CO₂ uptake, and CH₄ and N₂O emissions within a coastal rice ecosystem (tidal-influenced and saline), and explored the microbial mechanisms responsible for GHGs mitigation. The study was conducted over 2 years on Chongming island, Shanghai, China. Here, the use of 6‰ saltwater for irrigation in sea rice cultivation led to an increase in net ecosystem CO₂ exchange and gross primary productivity, mitigated CH₄ and N₂O emissions through tidal saltwater treatment with a projected decrease in CH₄ emissions during the tillering stage, and was accompanied by a marked upregulation of the AcsB gene associated with CO₂ fixation as well as the PmoA and Nirk2 genes involved in CH₄ and N₂O oxidation. Despite a decrease in plant height, this approach promoted tillering, thereby increasing shoot dry mass and ultimately maintaining rice yields without reduction. The study further revealed the combined CO₂-equivalent emissions of CO₂, CH₄, and N₂O during the tidal saltwater irrigation period, with reductions ranging from 22 % to 39 % for two different sea rice varieties. Therefore, the experimental simulation of tidal saltwater in a coastal rice system demonstrated its ability to sustain yield, increase carbon uptake and mitigate GHG emissions, although the effects were not statistically significant. We suggest that coastal rice cultivation using natural tidal irrigation is an effective approach to strengthen the ecological of coastal tidal mudflats by reducing GHG emissions and enhancing rice yields.